This invention relates to insulated gate field effect transistors and particularly to improved means for making ohmic contacts of good integrity to silicon surface regions thereof.
In conventional insulated gate field effect transistors, commonly known by the acronym IGFET, the source and drain electrodes usually comprise a low melting point metal such as aluminum, which has been deposited on the surface of the source and drain regions making ohmic contact therewith. While aluminum may be satisfactory for this purpose in such devices that have relatively deep source and drain regions, that is, depths greatly exceeding 1 micron, it has not proven to be satisfactory for making ohmic contact to source and drain regions which are rather shallow, such as 1 micron or less in depth. A serious problem encountered with aluminum or other low melting point metal is that concerning the strong tendency for the metal to diffuse through the shallow depth of the semiconductor regions and thereby cause the underlying semiconductor junction to be short circuited. Also, in IGFETS employing a polysilicon gate it is not always a routine matter to make ohmic contact to the polysilicon gate which is of good integrity in the first instance and which retains it thereafter.